Electric double layer capacitors (EDLCs) are power storage devices that store electric power by using a phenomenon in which ions in an electrolytic solution form an electric double layer due to physical adsorption to an electrode when an electric field is applied to the electrolytic solution. As compared with secondary batteries such as lithium-ion batteries that generate electricity by a chemical reaction, the EDLCs have a higher charging and discharging speed and thus are widely used in uninterruptible power supply systems (UPSs), for example. In recent years, social demands for environmental consciousness and energy efficiency have become more severe, and EDLCs with the above-mentioned properties have been more widely, increasingly demanded as temporary electric power storage apparatuses for energy recovery and use of the energy as auxiliary electric power in automobiles and the like and for wind power energy, and as electric power supplies for copy machines or similar machines to return from a standby condition. For the reasons, EDLCs are one of the most attractive power storage devices.
In recent years, electrochemical devices including batteries and capacitors are required to have much higher output density and energy density, and organic electrolytic solutions (nonaqueous electrolytic solutions) have been more widely used than aqueous electrolytic solutions from the viewpoint of voltage endurance.
Examples of known and widely used organic electrolytic solutions include an electrolytic solution prepared by dissolving a solid ammonium salt (electrolyte) such as a linear aliphatic ammonium salt for example, a tetraethylammonium salt and a triethylmethylammonium salt in an organic solvent such as propylene carbonate (Patent Literature 1); and electrolytic solutions prepared by dissolving a cyclic aliphatic ammonium salt such as an N-ethyl-N-methyl pyrrolidinium salt (Patent Literature 2) or a spiro-(1,1)-bipiperidinium salt (Patent Literature 3) in an organic solvent such as propylene carbonate. However, the electrolytic solutions have insufficient durability when used in electric double layer capacitors. One of the reasons for the insufficient durability is decomposition of a carbonate solvent used as the solvent. When the carbonate solvent is decomposed, carbon monoxide or carbon dioxide is generated and reacts with the electrolyte. The reaction products presumably further cause complicated decomposition reactions, deteriorating the performance of the electrolytic solution, for example.